This invention relates generally to a solar energy conversion system with parabolic trough reflectors and arrays of solar cells; more specifically, the invention relates to a system with an innovative structural framework which maintains the position and alignment of the parabolic trough reflectors relative to the solar cell arrays as the entire system rotates about a vertical axis to track the sun.
Solar cells are photovoltaic devices which convert sunlight directly into electrical energy. The past few decades have seen significant increases in solar cell efficiency and dramatic decreases in the cost of some types of cells. However, in spite of cost reductions and efficiency improvements, it is often the case that electricity generated by solar cells is provided to customers at real (unsubsidized) costs that are greater than the cost of electricity generated by conventional means.
Three major problems prevent solar-cell-based systems from more effectively penetrating the energy marketplace. First, even though the cost of solar cells has decreased, the cells still represent a major cost driver for installations that use them. Second, even though the efficiencies of some types of solar cells have increased, at least 75 to 80% of the solar energy incident on the most commonly used cells is not converted to useful energy. Third, most solar-cell-based installations are individually specialized designs for homes, businesses, airports, etc. This latter situation leads to site-specific design costs and high per-kilowatt installation and maintenance costs.
The objective of this invention is to provide a practical solar energy conversion system which successfully addresses the three problems mentioned above. This invention accomplishes its objective by offering a system incorporating important extensions and improvements of previously-revealed concentrated solar power (CSP) technology.
Regarding the problem of solar cells as a major cost driver for solar-cell-based systems, it is well known that parabolic trough reflectors can be used to collect solar energy from a large area and focus it onto a much smaller area, with energy concentration ratios (ratio of trough reflector width to solar image width) in the range from 20 to 50 easily achieved. When used in a solar-cell-based energy conversion system, parabolic trough reflectors can decrease—by a factor on the order of 20 to 50 or even more—the area, and thus the cost, of the solar cells required to generate a given amount of electrical power. In addition, technology advances related to back-contact silicon solar cells (U.S. Pat. Nos. 7,339,110 and 8,889,462) now offer the potential for high solar-to-electrical conversion efficiencies even for cells exposed to energy concentration ratios greater than 50. However, the use of parabolic trough reflectors in conjunction with solar cell arrays creates the need for active cooling of the cells and it presents very demanding orientation and alignment requirements for system components.
The present invention discloses a novel structural framework which interconnects a multiplicity of parabolic trough reflectors with corresponding arrays of solar cells and associated cooling apparatus. The framework accurately maintains the position and alignment of the reflectors relative to the solar cell arrays and the cooling apparatus as the entire system rotates to track the sun, thus providing a practicable solar energy conversion system with greatly reduced solar cell cost. In this way, the present invention addresses the first problem mentioned above.
Regarding the problem of low overall energy utilization in solar energy conversion systems, the system disclosed herein converts a portion of incident solar radiation directly to electricity. It also collects and stores the unconverted portion of the incident energy—that portion which would normally be discarded as waste heat—so that it can be used as needed in low-temperature applications.
In the present system, parabolic trough reflectors focus solar radiation onto solar cells positioned in or near the focal planes of the reflectors. Because of the high radiant flux produced near the focal planes, the irradiated cells must be actively cooled to prevent them from reaching temperatures which would significantly reduce their efficiency and their lifetime. The amount of thermal energy which must be removed from the cells is quite large, being at least three to four times the quantity of electrical energy generated by the cells. The energy conversion system disclosed herein provides a practical, efficient method for cooling the irradiated cells and for storing the waste heat. The cooling apparatus, which is an integral part of the aforementioned structural framework, removes low-grade waste heat (sensible fluid temperatures less than roughly 60° C.) from the solar cells and collects it in a cooling fluid. The system causes the cooling fluid to pass through, and discharge thermal energy to, a storage medium, which could be underground layers of gravel or rock. Energy transfer from the system to the storage medium can be direct, or it can be indirect, as through an intermediate heat exchanger. In either case, the accumulated thermal energy may be reclaimed from the storage medium as needed—after days, weeks, or even months of storage—for use locally in applications such as space heating and hot water heating for homes and businesses, or as process heat for industrial or agricultural operations. (The process of storing low-grade thermal energy for long periods of time is referred to as Seasonal Thermal Energy Storage (STES). STES technology has been proven to be commercially viable even in cold climates and at high latitudes, e.g. Drake Landing Solar Community in Alberta, Canada and Braedstrup Fjernvarm in Braedstrup, Denmark.) The low-grade thermal energy collected and stored by the present system has high value because it can replace the fossil fuels that are normally burned in low-temperature applications. It is also worth noting that roughly 20% of the electrical energy generated in industrially developed countries is used for space heating, hot water heating, and low-grade process heat. The thermal energy collected and stored by the presently disclosed system can therefore be used directly to replace a significant fraction of the electrical energy now generated in developed countries by conventional methods.
The system described herein is thus seen to be a dual-use system, generating both electricity and valuable low-grade thermal energy which can be stored for long periods of time. The system makes use of the complete spectrum of solar wavelengths, even those wavelengths which are energetically incapable of producing electrical current within a solar cell. The present invention addresses the second problem mentioned above by providing a practical solar energy conversion system with substantial energy utilization improvements over previously revealed systems.
Regarding the problem of size and site specificity of solar energy conversion systems relative to the size and location of their intended energy markets, this invention offers improvements and extensions of existing art as it pertains to CSP systems that use trough reflectors.
Existing CSP art includes two different types of trough reflector systems. The first type, used in most currently operating CSP systems, uses trough reflectors that are permanently oriented in either a North/South direction or an East/West direction. For these systems, each reflector rotates, or tilts, about its focal line in order to track the sun. Energy conversion systems of this type will be referred to herein as Tilting Trough Reflector (TTR) systems. TTR systems have three serious drawbacks. The first drawback is that, since each row of trough reflectors tilts in order to accomplish sun tracking, the reflector rows must be separated in order to avoid mutual shadowing at low sun angles. This leads to poor land area utilization, which in many currently operating TTR systems is less than 30%. Poor land area utilization means that these systems must be located in areas where average daily insolation is high and land costs are low. Such locations are usually far from the population and industrial centers that use the energy. The second drawback for TTR systems is that each row of reflectors requires its own individual drive and control mechanisms in order to track the sun. This adds considerable complexity to the overall system—complexity which increases installation and maintenance costs and decreases system reliability. The third drawback for TTR systems is that the tilting trough reflectors expose individual support members to directionally varying gravitational loads. This greatly increases the mechanical demands placed on the support structure, again increasing system cost and complexity.
A second type of trough reflector system, a type revealed in U.S. Pat. No. 4,159,629, avoids the inherent problems associated with TTR systems. That patent disclosed an energy conversion system wherein the rows of trough reflectors do not have a fixed orientation. Instead, all the rows of reflectors rotate in a horizontal plane about a common vertical axis as they track the sun. These non-tilting trough reflectors can be positioned side by side, thus providing efficient utilization of available surface area. Also, as noted in the above mentioned patent, all the reflectors can be moved with a single drive/control mechanism, thus providing significant system simplification. Finally, the non-tilting trough reflectors revealed in U.S. Pat. No. 4,159,629 provide directionally invariant loads to the reflector/receiver support structure.
Energy conversion systems which use non-tilting trough reflectors will be referred to herein as Vertical Axis Trough Reflector (VATR) systems because sun tracking is accomplished by simultaneously rotating all the rows of trough reflectors about a central vertical axis. The present invention is based on the VATR system concept. When the structural framework disclosed herein is coupled with the VATR system concept, the resulting energy conversion system is modular and scalable, and because of its high energy utilization capability, the system is practical even in locations where latitude or weather patterns reduce the total amount of solar energy available in a given year. Most importantly, because the system makes efficient use of available area, it can be deployed in close proximity to population and industrial centers where the energy is used. This latter characteristic is important because the valuable low-grade thermal energy provided by the system cannot be efficiently transported over long distances. It must be used locally. The modularity, scalability, and site adaptability of the presently disclosed system successfully address the third problem mentioned above.